Ultimate Guide to AC Contactor Abnormal Noise: From Electromagnetic Noise to Mechanical Vibration, Accurate Diagnosis and Handling

In electrical control systems, AC contactors are among the commonly used electrical components, and they are also a common source of various electrical faults. After long-term use—especially in harsh environments with high levels of dust—AC contactors may sometimes emit squeaking or rattling noises after being pulled in and held. The causes of this phenomenon are analyzed as follows.

Squeaking Noise After Pull-In and Holding

A fully functional AC contactor makes no noise when energized and pulled in. If a squeaking noise occurs during pull-in, the causes include: dirt on the surfaces of the moving iron core and static iron core; uneven force on the compression spring for resetting the moving iron core; or unsmooth movement path of the moving iron core.

These issues result in poor contact and gaps between the surfaces of the moving iron core and static iron core, which increases the magnetic resistance of the magnetic circuit and reduces the magnetic attraction force. To counteract this, the current in the coil increases to prevent the magnetic attraction force from decreasing, and this adjustment process repeats continuously. The squeaking noise is exactly the resonance caused by the coil current noise and the vibration of the reset compression spring— the larger the gap between the moving iron core and static iron core, the louder the squeaking noise.

Consequences

a. The AC contactor coil may burn out.

b. Poor contact may occur between the main and auxiliary contacts. In particular, the main contacts bear a large load, making them prone to arc generation, which can burn the main contacts or cause uneven adhesion. Additionally, phase loss may occur, leading to phase-loss operation of the three-phase load (e.g., an electric motor) and even burning of the three-phase load. If the auxiliary contacts are used in other branches, the normal operation of those branches will be affected.

Therefore, once an AC contactor emits a squeaking noise, it should be addressed immediately.

II. Rattling Noise During Pull-In

When an AC contactor pulls in, a rattling noise occurring 100 times per second is caused by an open circuit in the short-circuit ring of the contactor’s static (or moving) iron core.

Alternating current with a frequency of 50 Hz crosses zero 100 times per second. At the zero-crossing point, the magnetic force generated by the closed magnetic circuit formed by the moving and static iron cores also drops to zero. The function of the short-circuit ring (installed on the static or moving iron core) is to generate a counter electromotive force when the alternating current crosses zero. This counter electromotive force induces a current in the short-circuit ring, and the current produces a magnetic field that keeps the moving and static iron cores pulled together.

Once the short-circuit ring has an open circuit, its maintaining function is lost. At the zero-crossing point, the moving iron core is released under the action of the reset compression spring; after the zero-crossing point, the moving and static iron cores pull together again. This cycle repeats continuously, producing a rattling noise 100 times per second—which is the impact sound generated when the moving and static iron cores pull together.

Consequences

The connected three-phase load will be in a state of repeated starting and stopping, which easily damages the load. The consequences caused by auxiliary contacts are the same as those mentioned above.

In such cases, replace the AC contactor, or temporarily use a copper wire to make a short-circuit ring as a substitute.